Forecasts of Intraseasonal Periods of Tropical Cyclone Inactivity Over the Tropical Western North Pacific Ocean

During June-October, the average time period between tropical cyclone formations over the tropical western North Pacific Ocean is 6.7 days (one climatological period). However, extended periods of inactivity, defined by at least 18 days or three climatological periods between tropical cyclone formations, typically occur during each season. Identification of the large-scale circulation characteristics associated with periods of inactivity and the subsequent prediction of inactive periods are important for planning maritime activities over one- to three-week periods during the peak tropical cyclone season.

The re-analysis data set from the European Center for Medium-Range Weather Forecasts (ECMWF) during 1979-1993 has been used to construct characteristic large-scale circulation patterns over the tropical and subtropical western North Pacific Ocean. Four principal modes of variability have been identified as having an influence on tropical cyclone activity over the western North Pacific. One mode represents changes in the intensity and eastward extension of the monsoon trough through the Philippine Sea. Although the space and time characteristics of this mode indicate that it is likely associated with the intraseasonal Madden-Julian Oscillation, there is no significant correlation between this mode alone and tropical cyclone inactive periods. A second mode represents “trade-wind surges” that result in a weakened monsoon trough over the western North Pacific. Other modes represent smaller space and shorter time scale changes in the large-scale circulation. The amplitude of these modes appear related to synoptic-scale features in the midlatitudes and to the impact of previous tropical cyclones moving in the large-scale circulation.

The onset of inactive periods does not appear to be related to one dominant mode of circulation variability. Rather, variability over a wide range of space and time scales contribute to form circulation patterns that are not favorable for tropical cyclone formation over the western North Pacific Ocean. A canonical correlation analysis is used to define combinations of circulation modes that lead to inactive periods. Identification of the combinations and their importance for forecasting periods of tropical cyclone inactivity 10 to 15 days in advance will be presented.